Two major application areas of ethylene/alpha-olefin copolymers high density polyethylene (HDPE) and linear low density polyethylene (LLDPE) resins, include film and injection-molded articles.
When HDPE or LLDPE resins are fabricated into injection-molded products, it is imperative to assure that the manufacture of such products is not accompanied by warping or shrinking. As is known to those skilled in the art, the degree of warping or shrinking can be predicted from the molecular weight distributions of the resins. Resins having relatively narrow molecular weight distribution produce injection-molded products exhibiting a minimum amount of warping or shrinkage. Conversely, resins having relatively broader molecular weight distributions, produce injection-molded products more likely to suffer from warping or shrinkage.
One of the measures of the molecular weight distribution of an LLDPE or a HDPE resin is the melt flow ratio (MFR), which is the ratio of the high-load melt index (I.sub.21) to the melt index (I.sub.2) for a given resin: EQU MFR=I.sub.21 /I.sub.2
The melt flow ratio is an indication of the molecular weight distribution of the polymer: the higher the MFR value, the broader the molecular weight distribution. Resins having relatively low MFR values, e.g., of about 20 to about 30, have relatively narrow molecular weight distributions. LLDPE resins having such relatively low MFR values produce warpage-free injection-molded articles and film with better strength properties compared to resins with high MFR values.
Another important property of an ethylene/alpha-olefin copolymerization catalyst is the ability to effectively copolymerize ethylene with higher alpha-olefins, e.g., C.sub.3 -C.sub.10 alpha-olefins, and to produce LLDPE resins with low densities. Such resins have important advantages. They are used to produce LLDPE film which has excellent physical properties, and, in particular, is substantially more resistant to tearing and puncturing than a film made from similar resins of higher densities. This property of the catalyst is referred to as "alpha-olefin incorporation property" and is usually evaluated by determining the amount of an alpha-olefin (e.g., 1-butene, 1-hexene or 1-octene) required in the polymerization process to produce an ethylene copolymer with a given density. The lower is the amount of the alpha-olefin required to produce a resin of a given density, the higher is the production rates and, therefore, the lower is the cost of producing such a copolymer. High alpha-olefin incorporation ability is especially important in the gas-phase fluidized-bed processes, because relatively high concentrations of an alpha-olefin in the fluidized-bed reactor may cause poor fluidization due to resin stickiness. To avoid it, production rates must be significantly reduced. Consequently, catalyst compositions with a relatively high alpha-olefin incorporation ability avoid these problems and are more desirable.